FIG. 1 illustrates an optical communications system including a transmitter 1, an optical fiber link 5 and a coherent optical receiver known, for example, from Applicant's co-pending U.S. patent application Ser. No. 11/551,367 filed Oct. 20, 2006 which issued to U.S. Pat. No. 7,684,712 on Mar. 23, 2010, U.S. patent application Ser. Nos. 11/315,342 and 11/315,345, both of which were filed Dec. 23, 2005 and which issued to U.S. Pat. No. 7,627,252 on Dec. 1, 2009 and U.S. Pat. No. 7,532,822 on May 12 2009, and U.S. patent application Ser. No. 11/950,585 filed Dec. 5, 2007 which issued to U.S. Pat. No. 8,005,368 on Aug. 23, 2011. The transmitter 1 may operate in the manner closely similar to that known, for example, from U.S. Pat. No. 7,382,985, which issued Jun. 3, 2008. The entire contents of U.S. patent application Ser. Nos. 11/551,367, 11/315,342, 11/315,345 and 11/950,585 are incorporated herein by reference.
As may be seen in FIG. 1, an inbound optical signal is received through the optical link 2, split into orthogonal received polarizations polarization signals (X and Y) by a Polarization Beam Splitter 4, and then mixed with a Local Oscillator (LO) signal 6 by a conventional 90° optical hybrid 8. The composite optical signals emerging from the optical hybrid 8 are supplied to respective photodetectors 10, which generate corresponding analog electrical signals. The photodetector signals are sampled by respective Analog-to-Digital (A/D) converters 12 to yield raw multi-bit digital signals IX, QX and IY, QY corresponding to In-phase (I) and Quadrature (Q) components of each of the received polarizations (X, Y).
From the A/D converter 12 block, the respective n-bit signals IX, QX and IY, QY of each received polarization are supplied to an agile signal equalizer 14 which operates to compensate chromatic dispersion and polarization rotation impairments. In general, the signal equalizer 14 comprises a respective dispersion compensation block 16 for each of the X- and Y-polarizations, and a polarization compensation block 18. The dispersion compensation blocks 16 have a width sufficient to enable compensation of moderate-to-severe dispersion (e.g. on the order of 10000 ps/nm) based on a set of dispersion compensation coefficients 20, and generate respective intermediate vectors {TAX} and {TAY}. These intermediate vectors {TAX} and {TAY} are then input to the polarization compensation block 18, which uses a set of polarization compensation vectors HXX, HXY, HYY and HYX to impose a phase rotation which compensates polarization impairments of the optical signal, and so de-convolve the transmitted symbols from the raw digital sample streams IX, QX, and IY, QY generated by the A/D converters 12. The compensated signals 22 output from the equalizer 14 represent multi-bit estimates X′(n) and Y′(n) of the symbols encoded on each transmitted polarization of the received optical signal. These symbol estimates X′(n), Y′(n), are supplied to a carrier recovery block 24 for LO frequency control, symbol detection and data recovery, such as described in Applicant's co-pending U.S. patent application Ser. No. 11/366,392 filed Mar. 2, 2006.
The recovered data 26 output from the carrier recovery block 24 comprises respective decision values X(n), Y(n) of the symbols modulated onto each transmitted polarization of the received optical signal. Note that in the present description, “n” is an index of each transmitted symbol. These decision values X(n), Y(n) are passed to respective X- and Y-polarization decoding blocks 28 (such as, for example, a Forward Error Correction (FEC) block, Viterbi decoder, Low Density Parity Check (LDPC) decoder, Turbo decoder etc.) for error correction and data recovery.
In general, the decision values 26 X(n), Y(n) output from the carrier recovery block 24 have the same format as the originally transmitted symbols modulated onto each transmitted polarization. If desired, the decision values X(n), Y(n) may be demultiplexed (or otherwise processed) to obtain a set of recovered signals corresponding to the original signals encoded in the transmitted symbols. For example, Applicant's U.S. Pat. No. 7,522,841, which issued Apr. 21, 2009, teaches an arrangement in which four parallel tributary signals are bit-wise multiplexed to yield a pair of 2-bit symbol streams. Each symbol stream is then modulated and transmitted on a respective polarization using a Quadrature Phase Shift Keying (QPSK) or Quadrature Amplitude Modulation (QAM) modulation scheme. In such a case, the decision values 26 X(n), Y(n) output by the carrier recovery block 26 would be 2-bit estimates of the symbols modulated on the respective transmitted polarization. These two-bit estimates may be decomposed to yield four parallel bit-streams respectively corresponding with the original tributary signals.
A Least Mean Squares (LMS) update block 30 computes the polarization compensation vectors HXX, HXY, HYY and HYX based on the intermediate vectors {TAX} and {TAY}, as well as symbol phase φX, φX and error eX, eY information received from the carrier recovery block 24. As described in Applicant's co-pending U.S. patent application Ser. No. 11/950,585, the polarization compensation vectors HXX, HXY, HYY and HYX are updated at a sufficiently high rate to enable tracking, and therefore compensation, of polarization rotation transients at speeds on the order of 50 kHz.
In the coherent optical receiver of FIG. 1, polarization compensation is performed in the Jones Matrix domain using polarization compensation vectors HXX, HXY, HYY and HYX computed using an LMS algorithm, but other methods may equally be used. Recursive least squares, and constant modulus are two other examples of algorithms that can be used for calculation of the polarization compensation parameters. The polarization compensation can be implemented, for example, in the frequency domain, the time domain, the Jones Matrix domain, or combinations of such operations.
The coherent optical receiver of FIG. 1 is capable of data recovery from a high-speed optical signal (e.g. symbol rates above 10 Gbaud) composed of two independently modulated orthogonal polarizations, even in the presence of moderate to severe ISI due to chromatic dispersion (CD) and polarization mode dispersion (PMD), and polarization transients on the order of 50 kHz.
As will be appreciated from the above description, the decision values 26 X(n), Y(n) contain residual noise, which is intended to be compensated by the decoding block(s) 28. Under normal conditions, the polarization compensation block 18 effectively removes any correlation between the X- and Y-polarization decision values X(n), Y(n). This implies that the residual noise in the X- and Y-polarization decision values X(n), Y(n) is also un-correlated.
As is known in the art, Polarization Dependent Loss (PDL) can produce a state in which the X- and Y-polarizations of the received optical signal are not orthogonal. Under some circumstances, this can lead to a condition in which a residual noise correlation exists between the X- and Y-polarization decision values 26 X(n), Y(n). If the decoding blocks 28 are designed assuming that the noise is un-correlated, the presence of residual noise correlation can result in a performance penalty of up to about 2 dB. On the other hand, designing the decoding blocks 28 to perform error correction in the presence of noise correlation dramatically increases the complexity (and thus cost) of the decoding blocks and/or requires increased signal overhead for error correction, both of which are undesirable.